An unexpected detection of bifurcated blue straggler sequences in the young globular cluster NGC 2173

TL;DR

This paper reports the discovery of two distinct blue straggler star sequences in the young cluster NGC 2173, challenging previous assumptions that such bifurcations are solely due to stellar collisions in dense clusters.

Contribution

It presents the first detection of bifurcated blue straggler sequences in a young, low-density cluster, suggesting binary evolution as the likely formation mechanism.

Findings

Bifurcated blue straggler sequences detected in NGC 2173.

Stellar collisions are unlikely to explain the bifurcation.

Binary evolution is proposed as the primary formation process.

Abstract

Bifurcated patterns of blue straggler stars in their color--magnitude diagrams have atracted significant attention. This type of special (but rare) pattern of two distinct blue straggler sequences is commonly interpreted as evidence of cluster core-collapse-driven stellar collisions as an efficient formation mechanism. Here, we report the detection of a bifurcated blue straggler distribution in a young Large MagellanicCloud cluster, NGC 2173. Because of the cluster's low central stellar number density and its young age, dynamical analysis shows that stellar collisions alone cannot explain the observed blue straggler stars. Therefore, binary evolution is instead the most viable explanation of the origin of these blue straggler stars. However, the reason why binary evolution would render the color--magnitude distribution of blue straggler stars bifurcated remains unclear.